Improvements in LC/electrospray ion trap mass spectrometry performance using an off-axis nebulizer

Charged residues from the electrospray process have been hypothesized to li mit the sensitivity and dynamic range of an ion trap mass spectrometry oper ation. Incorporation of an off-axis nebulizer (positioned 90-95 degrees fro m the sampling orifice) was found to drastically reduce the detrimental eff ects caused by the charged particles or droplets compared to typical on-axi s nebulization configurations (spraying 10-20 degrees from sampling orifice s). The off-axis nebulizer reduced total ion currents that enter the ion tr ap (through the reduction of charged residues) by a factor of 5-7 while res ulting in an increase of analyte [M + H](+) signal by a factor of 6 compare d to an on-axis sprayer at now rates of 20 mu L/min. At higher now rates (e .g., 800 mu L/min) these enhancements are more evident. At flows greater th an 200 mu L/min, off-axis nebulization reduced total. ion current that ente rs the ion trap by a factor of 30 and resulted in a factor of more than 20 increase in [M + H](+) signal relative to on-axis nebulization. Incorporati on of the off-axis nebulizer improved the detection limit and precision for determination of dihydroxyvitamin D-3 in plasma compared to on-axis nebuli zation. The LC/MS/MS detection limits obtained for the off-axis nebulizer o n the ion trap was within a factor of 2 from the detection limit determined by the triple quadrupole. The relative standard deviation of the dihydroxy vitamin D-3 determination was less than 8% for both off-axis ion trap and t riple-quadrupole determinations.